Abstract

This paper describes photochemical effects observed during two-photon 1S–2S excitation of atomic hydrogen in flames using 243-nm laser radiation. An I4 intensity dependence is observed in regions of the flame where the natural atomic concentration is low, suggesting an I2 photochemical production mechanism, which we believe is due to two-photon excitation of water molecules, which then predissociate to form H and OH fragments. In a measurement of OH created in the flame by the 243-nm beam, we observe the same I2 intensity dependence with the laser detuned from the atomic hydrogen 1S–2S resonance, but an apparent I3,4 dependence is observed when the laser is tuned to the resonance. We believe that a second photochemical mechanism contributes at the resonance, namely, two-photon excitation of H, followed by collisional energy transfer to water molecules, which then fall apart into H and OH fragments. We model this process and show that a combination of I2 and I4 dependences can lead to an intensity dependence that mimics a single I3,4 dependence over a limited range of intensities.

Dependences of the atomic hydrogen two-step fluorescence signal on 243-nm pulse energy measured in atmospheric pressure hydrogen–oxygen flames: circles, 1.3 mm above the burner in a rich (equivalence ratio 1.2) flame, triangles, 13 mm above the burner in a lean (equivalence ratio 0.8) flame. The dashed line passing through the circles has a slope of 2, consistent with an I2 intensity dependence of the two-photon excitation process, and the dashed line passing through the triangles has a slope of 4, suggesting an additional I2 intensity dependence for a photochemical production mechanism.

Measurement of the OH emission signal as the 243-nm pump beam was scanned across the atomic hydrogen 1S–2S transition. Top: OH fluorescence signal excited by the 281-nm probe beam; the dashed line represents the nascent OH fluorescence signal observed with the 243-nm beam blocked. Bottom: OH emission observed with only the 243-nm beam present. The same vertical units apply to both parts of the figure.

Dependences of the OH fluorescence probe signal on the pulse energy of the 243-nm pump beam: circles, 243-nm wavelength detuned from the atomic hydrogen 1S–2S transition; the dashed line has a slope of 2.0, consistent with an I2 intensity dependence for the photochemical creation mechanism; triangles, 243-nm wavelength tuned onto the 1S–2S transition; the dashed line has a slope of 3.4, suggesting a higher-order photochemical creation mechanism; squares, simulation of on-resonance signal (displaced horizontally for clarity).

Dependence of the 656-nm atomic hydrogen emission on the pulse energy of a 243-nm beam tuned to the atomic hydrogen 1S–2S transition; the dashed line, with a slope of 6.4, represents a linear fit to the data points.